In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor's surface either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor's surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor's surface onto the paper or intermediate medium still in the pattern of the image. A set of fuser rolls or belts, under heat, melts and fixes the toner in the paper subsequent to transfer, producing the printed image.
The electrostatic printing process, therefore, comprises an intricate and on-going series of steps in which the photoconductor surface is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places, at the appropriate times is critical to making the process work.
After the image is transferred to the paper or other recording medium, it goes to the fuser where the paper is moved through a nip where it is heated and pressed. This melts the thermoplastic portion of the toner, causing it to bond with the fibers of the paper, thereby fixing the image onto the paper or recording medium. While this is an effective way of fixing the toner image on the paper's surface, it carries with it some problems. A common problem with fusing mechanisms is the creasing or "treeing" of the print media as it passes through the fuser nip. Several factors, including environment, relative humidity, media type, entry conditions, and nip mechanics, can affect the tendency of a fuser to tree media. Regardless of the cause, creased, wrinkled or "treed" pages result in lost time, lost paper, and lost patience, as printing operations have to be repeated over again in order to get a non-creased product. While the issue of wrinkling, creasing and treeing has been addressed extensively in the fuser roll context, because the mechanics are different and somewhat more intricate, it has not been addressed extensively in the context of a belt fuser mechanism.
U.S. Pat. No. 5,355,204, Aoki, issued Oct. 11, 1994, describes a mechanism which is said to minimize creasing of the belt in a belt fuser mechanism. A creased fuser belt (as opposed to creased print media) is said to result in uneven fusing of the print media. The desired result is accomplished by crowning the heater (i.e., forming a convex surface on the heater) and inversely crowning (i.e., forming a complimentary concave surface) the backup roll such that they fit together. It will be noted that this is the opposite approach of the present invention in which it is important to form a concave surface on the heater and, therefore, on the fuser belt.
U.S. Pat. No. 5,450,181, Tsukida, et al., issued Sep. 12, 1995, which provides a fixing roller mechanism which is said to minimize print media wrinkling without forming corrugated edges on the printed page. The described device utilizes a fixing roller which basically has an inverse crown (i.e., concave) shape, but which moves the largest roller diameter in from the ends of the roll toward the center (see FIG. 3 of the patent). The entire focus of the structure is to avoid stretching the edges of the print media in order to avoid corrugation of the printed page.
U.S. Pat. No. 4,042,804, Moser, issued Aug. 16, 1977, describes a fuser roll mechanism which is said to minimize paper wrinkling. The fuser roll is structured such that the printed page moves through the fusing nip faster at its edges than at its center. The core of the fuser roll has a smaller diameter at its center than at its edges (i.e., a concave structure). An elastomeric outer layer is placed on the fuser roll core such that the finished roll has a uniform diameter across its length (i.e., the elastomeric layer is thicker at the center than at its ends). Thus, the overall structure of the roll itself is cylindrical, not concave.
U.S. Pat. No. 3,884,623, Slack, issued May 20, 1975, describes a fuser roll which is tapered in its diameter from its ends towards its center (i.e., a concave structure). This structure is said to minimize wrinkling in the printed pages produced.
U.S. Pat. No. 4,594,068, Bardutzky, et al., issued Jun. 10, 1986, describes a fuser roll structure which utilizes a concave-shaped roll core and a complimentary elastomeric coating, which is thicker in the middle and thinner at the ends of the roll, such that the overall coated fuser roll is uniform in diameter along its length. This roll structure is said to result in a higher through-put speed for the printed page at the roll ends than at the roll center, thereby stretching the printed page and minimizing wrinkling.
U.S. Pat. No. 5,195,430, Rise, issued Mar. 23, 1993, describes a fuser roll structure which is said to eliminate flexing of the fuser roll during use, thereby avoiding uneven fixing of the printed page. The patent teaches that crowning of fuser rollers is undesirable since it leads to wrinkling of the printed pages because of the velocity differences at various points along the fusing nip. This patent addresses this issue by utilizing a fixing roller having a metallic roller core which is crowned at the center (i.e., a convex shape); the roller is covered with an elastomeric material which is thinner at the middle than at the ends of the roll such that the overall diameter of the roll across its length is constant. This structure is said to minimize roller flexing and paper wrinkling.
U.S. Pat. No. 4,961,704, Nemoto, et al., issued Oct. 9, 1990, describes a mechanism for minimizing meandering of the printed page through the fuser nip, particularly when the printer is starting up. This is said to be accomplished by utilizing a fuser roller which allows the user to change the end pressure of the roller as needed, starting with higher pressures at the ends of the rollers. This pressure differential changes the velocity of the paper at various points along the nip, thereby keeping the paper moving through the nip straight.
U.S. Pat. No. 4,930,202, Yano, issued Jun. 5, 1990, describes fixing rollers which have a non-uniform diameter across their length; the rollers either crown at their center or at their ends. The shaft through the roller is bent to parallel the surface shape of the roller. This structure is said to decrease paper wrinkling and bending of the roller shaft during use.
U.S. Pat. No. 4,872,246, Yano, issued Oct. 10, 1989, describes fixer rolls which have a larger diameter at their ends than at their center (i.e., the rolls have a concave shape). The roll body is utilized on a curved shaft and this structure is said to minimize wrinkling of the printed page. See also, U.S. Pat. Nos. 4,803,877 and 4,870,731.
U.S. Pat. No. 3,999,038, Sikes, Jr., et al., issued Dec. 21, 1976, describes a fuser roll having an hour-glass shape (i.e., a concave structure) wherein the diameter of the ends of the roll is larger than the diameter of the center of the roll. This structure is said to reduce wrinkling of the printed page, especially in duplex operations. The patent suggests that the mechanism of action is that the paper velocity through the fusing nip is greater at the ends of the roll than at the middle of the roll, thereby stretching out any wrinkles formed.
U.S. Pat. No. 4,008,955, Bar-on, issued Feb. 22, 1977, describes a fuser roll structure which is said to minimize wrinkling of the printed pages formed. This is accomplished by placing rings at the ends of the backup roller, rather than on the fuser roller. When this backup roller is used in combination with a cylindrical fuser roll, the velocity of the paper through the nip at the ends of the nip is said to be greater than the velocity at the center of the nip, thereby minimizing wrinkling of the printed page.
U.S. Pat. No. 4,253,392, Brandon, et al., issued Mar. 3, 1981, describes a fuser roll having adjustable ends which allow the end diameter of the roll to be increased relative to the center diameter. In high humidity conditions, the roll can be made concave by increasing the diameters of the roll ends, which is said to eliminate wrinkling of printed pages moving through the nip. In low humidity conditions, the adjustable ends are used to make the roll cylindrical, thereby eliminating the smearing problem which is said to occur with concave fuser rolls.
U.S. Pat. No. 5,689,789, Moser, issued Nov. 18, 1997, describes a fuser roll configuration the purpose of which is to create a constant nip velocity across the length of the nip. This is said to be accomplished by crowning the fuser roller (i.e., making it thicker in the middle than on the ends; a convex structure) so that it forms a uniform nip thickness when compressed against the backup roller in use. This approach is the opposite of many of the other prior art references and of the present invention, the entire purpose of which is to create velocity differentials for the printed page at various points in the nip.
U.S. Pat. No. 5,655,201, Islam, et al., issued Aug. 5, 1997, describes a fuser roll structure which is used to fuse migration imaging members, rather than paper. The fixing roller is structured such that it contacts the backup roller only at its edges in order to minimize stresses at the middle of the roll.
The prior art, discussed above, does not suggest the use of a concave fuser belt or present any approach to the issue of minimizing wrinkling of the printed page in a fuser belt context. The art clearly recognizes that the minimization of printed page wrinkling in a fuser roll system is desirable. This is addressed by the prior art, in fuser roll systems, by variously forming concave fuser rolls, convex fuser rolls, and perfectly cylindrical fuser rolls (i.e., by modifying the driven nip member). Further, while some of the art suggests that formulating a fuser roll system such that a higher paper velocity at the ends of the roll rather than the middle of the roll is desirable, some of the art suggests that what is required is a uniform paper velocity across the entire nip. The bottom line is that the prior does not teach or suggest a solution for the problem of paper wrinkling in a fuser belt system.
It has now been discovered that by using a fuser belt system which is configured such that the printed page moves through the nip at a speed which is greater at the ends of the nip than at the center of the nip, wrinkling of the printed page is avoided. Specifically, this can be achieved by utilizing a concave heater frame to hold the heater, thereby giving the fuser belt in use a concave shape (i.e., by modifying the non-driven nip member), together with a substantially cylindrical backup member. This results in a saddle-shaped fuser nip which provides the desired velocity differential. The invention is described in greater detail below.